The invention relates to a power amplifier arrangement with high current drive capability, particularly but not exclusively for transmission systems.
In several transmission systems, such as Asymmetric Digital Subscriber Line, video distribution and audio, there is a need to transmit signals with high crest factors (peak/rms). This high crest factor results from short signal peaks that rarely occur, but which force the driver to be supplied from a high voltage supply. This results in a high dynamic power consumption in the amplifier.
In the seventies amplifiers were developed with lower power consumption, more particularly the class G amplifiers working at two different supply voltages. They use one large output stage, biased by two different voltage supplies, whereby diodes are generally used to control the switchover from the low voltage to the high voltage supply. Such G class transistor amplifiers are amongst others disclosed in U.S. Pat. No. 3,772,606 and U.S. Pat. No. 4,100,501 .
In case the transistors of the classical G class amplifiers have to drive large currents, they may be replaced by Darlington pairs, such as for instance disclosed in the above-mentioned U.S. Pat No. 4,100,501. In this prior art document, the described push-pull variant comprises two Darlington pairs having together four transistors of the same conductivity type and two Darlington pairs having together four transistors of the conductivity type opposite to that of the first mentioned Darlington pairs. The transistors of the first and second Darlington pairs conduct during each positive half cycle of the input signal. The transistors of the third and fourth Darlington pairs conduct during each negative half cycle of the signal. The collectors of the two transistors of a first Darlington pair are connected to each other and to a positive D.C. high voltage supply. Similarly, the collectors of the two transistors of a fourth Darlington pair are connected to each other and to a negative D.C. high voltage supply. In this prior art embodiment the collectors of the two transistors of the second Darlington pair are also connected to each other and via a diode to a positive D.C. low voltage supply, while the two transistors of the third Darlington pair are connected to each other and via another diode to a negative D.C. low voltage supply. The emitter of the output transistor of the first, respectively fourth, Darlington pair is coupled to the collectors of both transistors of the second, respectively third, Darlington pair.
This prior art solution whereby each transistor of the original class-G amplifier is replaced by a Darlington pair, results in a high current amplification and consequently a high current drive capability, but at the same time in a large voltage drop across all Darlington pairs.
The invention seeks to provide an amplifier with high current drive capability with relatively low dynamic power consumption, but avoiding such a large voltage drop across the transistors.